1. Field of Invention
The present invention relates to a liquid crystal display device test circuit, a liquid crystal device incorporated with the same, and a test method for the liquid crystal device.
2. Related Art
Among a variety of liquid crystal display devices, especially a thin film transistor (TFT) liquid crystal display device is comprised of liquid crystal display elements arranged in matrix on a glass substrate, thin film transistors (TFTs) having their respective drains connected to the liquid crystal display elements, row lines each of which is commonly connected to gates of the TFTs in a row and is controlled by a row driver, and column lines each of which is commonly connected to sources of the TFTs in a column and is controlled by a column driver to receive multi-level voltage depending upon gradation of an image to be displayed, and typically, the TFTs, the row lines, the column lines, and other components are built up through the amorphous silicon process technology.
In general, after these primary components of the liquid crystal display elements are formed, a process of attachment of color filter glass is followed by injecting liquid crystal to complete the device, and thereafter, the finished device undergoes a lighting-up test. During the lighting-up test, a row decoder activates a target row line to turn on the TFTs, gates of which are connected to the target row line. And voltages depending upon the brightness of an image to be displayed are applied by the column driver to the data (column) lines to which sources of the TFTs are connected. The voltages are supplied through the TFT to a pixel electrode connected to the drain of the TFT, and thus, the liquid crystal is varied in transmissivity to give the desired display. Pass or fail on the lighting-up test is judged by visual inspections.
In this way, since the lighting-up test is carried out on the device completely finished, it is a sheer waste of the device manufacturing process when some defects are observed after all.
On the other hand, the recent development of the low temperature poly silicon (LTPS) process technology has boosted up numbers of circuits implemented on an array glass, and the boosting popularity of much larger liquid crystal display devices in the market, as well, will probably cause an increase in a rejection rate of the products far beyond the current figure.
Considering these situations, an improvement is disclosed where, for the device before completion, in order to perform a test of an image display, signals from all the points to be inspected are taken out to peripheral parts of an array substrate provided with a numerous analog switches, and an external test signal is applied to the device so as to detect off leak malfunction (see Patent Document 1 listed below).
Patent Document 1: Japanese Patent Preliminary Publication No. 2001-330639.
Such a prior art, however, utilizes two Off level voltages and a way of increasing leak current to detect off leak malfunction, and a minute current variation due to the off leak malfunction cannot be assuredly detected by this embodiment. In addition, a numerous terminals and analog switches must be provided in the peripheral parts of the liquid crystal display device for the purpose of detecting the malfunction betray the desires of downsizing and reduction of the space.